Re: [PATCH v2 3/3] gpiolib: pin GPIO devices in place during descriptor lookup

[Marek Szyprowski]

On 02.01.2024 16:59, Bartosz Golaszewski wrote:
> From: Bartosz Golaszewski <bartosz.golaszewski@xxxxxxxxxx>
>
> There's time between when we locate the relevant descriptor during
> lookup and when we actually take the reference to its parent GPIO
> device where - if the GPIO device in question is removed - we'll end up
> with a dangling pointer to freed memory. Make sure devices cannot be
> removed until we hold a new reference to the device.
>
> Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@xxxxxxxxxx>

This patch landed in linux-next as commit db660b9a9f86 ("gpiolib: pin 
GPIO devices in place during descriptor lookup"). Unfortunately it 
introduces a following lock-dep warning:

============================================
WARNING: possible recursive locking detected
6.7.0-rc7-00062-gdb660b9a9f86 #7819 Not tainted
--------------------------------------------
kworker/u4:2/27 is trying to acquire lock:
c13f4e1c (gpio_devices_sem){++++}-{3:3}, at: gpio_device_find+0x30/0x94

but task is already holding lock:
c13f4e1c (gpio_devices_sem){++++}-{3:3}, at: 
gpiod_find_and_request+0x44/0x594

other info that might help us debug this:
  Possible unsafe locking scenario:

        CPU0
        ----
   lock(gpio_devices_sem);
   lock(gpio_devices_sem);

  *** DEADLOCK ***

  May be due to missing lock nesting notation

4 locks held by kworker/u4:2/27:
  #0: c1c06ca8 ((wq_completion)events_unbound){+.+.}-{0:0}, at: 
process_one_work+0x148/0x608
  #1: e093df20 ((work_completion)(&entry->work)){+.+.}-{0:0}, at: 
process_one_work+0x148/0x608
  #2: c1f3048c (&dev->mutex){....}-{3:3}, at: 
__driver_attach_async_helper+0x38/0xec
  #3: c13f4e1c (gpio_devices_sem){++++}-{3:3}, at: 
gpiod_find_and_request+0x44/0x594

stack backtrace:
CPU: 0 PID: 27 Comm: kworker/u4:2 Not tainted 
6.7.0-rc7-00062-gdb660b9a9f86 #7819
Hardware name: Samsung Exynos (Flattened Device Tree)
Workqueue: events_unbound async_run_entry_fn
  unwind_backtrace from show_stack+0x10/0x14
  show_stack from dump_stack_lvl+0x58/0x70
  dump_stack_lvl from __lock_acquire+0x1300/0x2984
  __lock_acquire from lock_acquire+0x130/0x37c
  lock_acquire from down_read+0x44/0x224
  down_read from gpio_device_find+0x30/0x94
  gpio_device_find from of_get_named_gpiod_flags+0xa4/0x3a8
  of_get_named_gpiod_flags from of_find_gpio+0x80/0x168
  of_find_gpio from gpiod_find_and_request+0x120/0x594
  gpiod_find_and_request from gpiod_get_optional+0x54/0x90
  gpiod_get_optional from reg_fixed_voltage_probe+0x200/0x400
  reg_fixed_voltage_probe from platform_probe+0x5c/0xb8
  platform_probe from really_probe+0xe0/0x400
  really_probe from __driver_probe_device+0x9c/0x1f0
  __driver_probe_device from driver_probe_device+0x30/0xc0
  driver_probe_device from __driver_attach_async_helper+0x54/0xec
  __driver_attach_async_helper from async_run_entry_fn+0x40/0x154
  async_run_entry_fn from process_one_work+0x204/0x608
  process_one_work from worker_thread+0x1e0/0x498
  worker_thread from kthread+0x104/0x138
  kthread from ret_from_fork+0x14/0x28
Exception stack(0xe093dfb0 to 0xe093dff8)
...


Taking gpio_devices_sem more than once for reading is safe, but it looks 
that it needs some lock-dep annotations to to make it happy and avoid 
the above warning.


> ---
>   drivers/gpio/gpiolib.c | 40 +++++++++++++++++++++++-----------------
>   1 file changed, 23 insertions(+), 17 deletions(-)
>
> diff --git a/drivers/gpio/gpiolib.c b/drivers/gpio/gpiolib.c
> index 4c93cf73a826..be57f8d6aeae 100644
> --- a/drivers/gpio/gpiolib.c
> +++ b/drivers/gpio/gpiolib.c
> @@ -4134,27 +4134,33 @@ static struct gpio_desc *gpiod_find_and_request(struct device *consumer,
>   	struct gpio_desc *desc;
>   	int ret;
>   
> -	desc = gpiod_find_by_fwnode(fwnode, consumer, con_id, idx, &flags, &lookupflags);
> -	if (gpiod_not_found(desc) && platform_lookup_allowed) {
> +	scoped_guard(rwsem_read, &gpio_devices_sem) {
> +		desc = gpiod_find_by_fwnode(fwnode, consumer, con_id, idx,
> +					    &flags, &lookupflags);
> +		if (gpiod_not_found(desc) && platform_lookup_allowed) {
> +			/*
> +			 * Either we are not using DT or ACPI, or their lookup
> +			 * did not return a result. In that case, use platform
> +			 * lookup as a fallback.
> +			 */
> +			dev_dbg(consumer,
> +				"using lookup tables for GPIO lookup\n");
> +			desc = gpiod_find(consumer, con_id, idx, &lookupflags);
> +		}
> +
> +		if (IS_ERR(desc)) {
> +			dev_dbg(consumer, "No GPIO consumer %s found\n",
> +				con_id);
> +			return desc;
> +		}
> +
>   		/*
> -		 * Either we are not using DT or ACPI, or their lookup did not
> -		 * return a result. In that case, use platform lookup as a
> -		 * fallback.
> +		 * If a connection label was passed use that, else attempt to
> +		 * use the device name as label
>   		 */
> -		dev_dbg(consumer, "using lookup tables for GPIO lookup\n");
> -		desc = gpiod_find(consumer, con_id, idx, &lookupflags);
> +		ret = gpiod_request(desc, label);
>   	}
>   
> -	if (IS_ERR(desc)) {
> -		dev_dbg(consumer, "No GPIO consumer %s found\n", con_id);
> -		return desc;
> -	}
> -
> -	/*
> -	 * If a connection label was passed use that, else attempt to use
> -	 * the device name as label
> -	 */
> -	ret = gpiod_request(desc, label);
>   	if (ret) {
>   		if (!(ret == -EBUSY && flags & GPIOD_FLAGS_BIT_NONEXCLUSIVE))
>   			return ERR_PTR(ret);

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland